Robert F. Powers

Nitrogen mineralization along an altitudinal gradient: Interactions of soil temperature, moisture, and substrate quality

Net soil N mineralization was measured in situ for 2 consecutive years along a 2000-m altitudinal gradient encompassing six vegetation types in northern California. Both anaerobic and aerobic field incubations showed that soil temperature and moisture strongly controlled N release. In sealed container studies, N mineralization per unit of total Kjeldahl N (tkn) had a Q10 rate of 2 and varied between 5 and 38 g N/kg tkn when moisture was abundant (anaerobic incubation). However, aerobic mineralization rates fell to between 2 and 22 g N/kg tkn because of summer drought. Aerobic rates were greatest at mid-elevations (the mixed-conifer forest), and were reduced by cold temperatures at higher elevations and by soil drought at lower. The lowest rate found for any site was in a pine plantation scalped of topsoil a decade previously, during site preparation. Open-container incubations involving intact, 15-cm soil cores and ion-exchange resins produced mineralization rates correlated with potential growing days, but not with rates from sealed containers. The forest floor accounted for nearly all the net N mobilized on the coldest site, but only one-half to one-third of the N mobilized on warmer sites. Open containers have important advantages in assessing soil N dynamics.

Lethal soil temperatures during burning of masticated forest residues

Mastication of woody shrubs is used increasingly as a management option to reduce fire risk at the wildland-urban interface. Whether the resulting mulch layer leads to extreme soil heating, if burned, is unknown. We measured temperature profiles in a clay loam soil during burning of Arctostaphylos residues. Four mulch depths were burned (0, 2.5, 7.5 and 12.5 cm), spanning typical conditions at forested sites in northern California with dense pre-mastication shrub cover. Two soil moisture contents were compared at each fuel depth to simulate spring prescribed burning (moist soil) and late-season wildfire (dry soil). Maximum temperatures reached 600 ◦ C on the surface of dry soils and were 100-200 ◦ C lower for moist soil. Heating was extensive in dry soil for the two deepest mulch depths, exceeding the lethal threshold for plants (60 ◦ C) for a minimum of 7 h throughout the 10-cm soil profile. Minimal heat pulse was found with less mulch. Moist soil also dampened heat penetration; peak temperatures exceeded 60 ◦ C only to 2.5 cm in the soil profile for all but the deepest mulch layer. No adverse effects of burning on water repellency were found in dry or moist soil. The potential for biological damage from soil heating during fire exists following mastication, particularly in dry soil with a mulch depth of 7.5 cm or greater. Field projections indicate that up to one-fourth of treated areas with dense pre-mastication vegetation would surpass lethal soil temperatures during a surface wildfire.

Linkages between phosphorus transformations and carbon decomposition in a forest soil

Phosphorus mineralization is chemically coupled with organic matter (OM) decomposition in surface horizons of a mixed-conifer forest soil from the Sierra Nevada, California, and is also affected by the disturbance caused by forest harvesting. Solution13C nuclear magnetic resonance (NMR) spectroscopy of NaOH extracts revealed a decrease of O-alkyl and alkyl-C fractions with increasing degree of decomposition and depth in the soil profile, while carbonyl and aromatic C increased. Solid-state13C-NMR analysis of whole soil samples showed similar trends, except that alkyl C increased with depth. Solution31P-NMR indicated that inorganic P (P1) increased with increasing depth, while organic-P (Po) fractions decreased. Close relationships between P mineralization and litter decomposition were suggested by correlations between P1 and C fractions (r = 0.82, 0.81, −0.87, and −0.76 for carbonyl, aromatic, alkyl and O-alkyl fractions, respectively). Correlations for diester-P and pyrophosphate with O-alkyl (r = 0.63 and 0.84) and inverse correlations with aromatics (r = −0.74 and −0.72) suggest that mineralization of these P fractions coincides with availability of C substrate. A correlation between monoester P and alkyl C (r = 0.63) suggests mineralization is linked to breakdown of structural components of the plant litter. NMR analyses, combined with Hedley-P fractionation, suggest that post-harvest buildup of labile P in decomposed litter increases the potential for leaching of P during the first post-harvest season, but also indicates reduced biological activity that transports P from litter to the mineral soil. Thus, P is temporarily stored in decomposed litter, preventing its fixation by mineral oxides. In the mineral horizons,31P-NMR provides evidence of decline in biologically-available P during the first post-harvest season.

ON THE SUSTAINABLE PRODUCTIVITY OF PLANTED FORESTS

Planted forests have more than a millennium of history and represent the worlds best hope for meeting global wood requirements in the twenty-first century. Advances in genetic improvement, nursery practices, stand establishment, and tending, harvesting, and manufacturing have boosted plantation yields to a higher level than at any point in history. Despite this, forest managers face a mounting challenge to demonstrate that plantation productivity is sustainable. Tackling this challenge requires a sound understanding of the principles of forest productivity, how they apply to a developing plantation, and how they are influenced by management. In this paper criticisms of plantation forestry are discussed from the basis of world experience, and examples of productivity decline are described. Obvious declines are rare, and can be attributed to poor soil management. However, ambiguities exist and controversy will continue until sustainable productivity can be demonstrated conclusively. Proposed programs aim to provide the technical base needed for sound soil management and sustainable plantation productivity.

Effects of timber harvest following wildfire in western North America.

Timber harvest following wildfire leads to different outcomes depending on the biophysical setting of the forest, pattern of burn severity, operational aspects of tree removal, and other management activities. Fire effects range from relatively minor, in which fire burns through the understory and may kill a few trees, to severe, in which fire kills most trees and removes much of the organic soil layer. Postfire logging adds to these effects by removing standing dead trees (snags) and disturbing the soil. The influence of postfire logging depends on the intensity of the fire, intensity of the logging operation, and management activities such as fuel treatments. In severely burned forest, timing of logging following fire (same season as fire vs. subsequent years) can influence the magnitude of effects on naturally regenerating trees, soils, and commercial wood value. Removal of snags reduces long-term fuel loads but generally results in increased amounts of fine fuels for the first few years after logging unless surface fuels are effectively treated. By reducing evapotranspiration, disturbing the soil organic horizon, and creating hydrophobic soils in some cases, fire can cause large increases in surface-water runoff, streamflow, and erosion. Through soil disturbance, especially the construction of roads, logging with ground-based equipment and cable yarding can exacerbate this effect, increasing erosion and altering hydrological function at the local scale. Effects on aquatic systems of removing trees are mostly negative, and logging and transportation systems that disturb the soil surface or accelerate road-related erosion can be particularly harmful unless disturbances are mitigated. Cavity-nesting birds, small mammals, and amphibians may be affected by harvest of standing dead and live trees, with negative effects on most species but positive or neutral effects on other species, depending on the intensity and extent of logging. Data gaps on postfire logging include the effects of various intensities of logging, patch size of harvest relative to fire size, and long-term (10+ years) biophysical changes. Uncertainty about the effects of postfire logging can be reduced by implementing management experiments to document long-term changes in natural resources at different spatial scales.

Ammonia electrode analysis of nitrogen in Microkjeldahl digests of forest vegetation

Abstract Nitrogen content of forest vegetation is analyzed frequently by the microKjeldahl method. After digestion, nitrogen recovery is determined traditionally by distillation and titration‐processes that require considerable time and bench space. The ammonia electrode technique applied to Kjeldahl digests offers a four‐fold saving in analytic time per sample and yields results comparable to those derived by the traditional method. Concentrations of nitrogen can be measured accurately in digests containing as little as 0.18 mg nitrogen, if calibration standards and samples are treated identically.

Decomposition of small diameter woody debris of red fir determined by nuclear magnetic resonance

Abstract Red fir (Abies magnifica A. Murr.) woody debris decomposing for 17 years in untreated (Control) and nitrogen‐fertilized plus widely thinned (NT2) plots was examined by 13C nuclear magnetic resonance (NMR). Total carbon (C) and total N concentrations were also determined. Combined data of wood and bark showed correlations between carboxylic, aromatic, O‐alkyl and aliphatic C fractions, and C fractions with C/N, but phenolic and methoxyl correlations were non‐significant. Wood mass losses averaged 38% for both Controls and NT2. Bark mass losses were 61% for Controls and 66% for NT2, but these were not significantly different at p<0.05; bark sloughing added considerable variance. Wood in Controls decreased O‐alkyl (66 to 50%) and aromatic (16 to 13%), increased carboxyl (1.5 to 6.5%) and aliphatic (2.0 to 15.5%), and decreased Cm/Lm, i.e., carbohydrate/lignin monomers (2.78 to 1.82). In NT2 plots, open crowns allowed greater drying of the forest floor during warm, dry summers. The C/N averages were ...

DESIGNING LONG-TERM SITE PRODUCTIVITY EXPERIMENTS

Exploring the long-term sustainability of managed forest ecosystem using the field experimental approach, while recognised as being a research priority, is not a straightforward task. This is true of many long-term forestry trials despite the use of the scientific method, designed experiments, and the advantages of modern equipment. Managers and researchers have frequently underestimated the problems involved in dealing with long-term forest experiments.

The Key Roles of Four Experimental Forests in the LTSP International Research Program

Four Experimental Forests were pivotal in piloting the long-term soil productivity (LTSP) cooperative research program—one of the most successful and extensive collaborative science efforts yet undertaken by the USDA Forest Service. Launched on the Palustris, Challenge, Marcell, and Priest River Experimental Forests, LTSP traces to a seminal discussion during a field tour in central Louisiana in 1986. P. E. Avers, National Soils Program Leader in Washington DC, described to D. H. Alban and R. F. Powers a problem arising from the National Forest Management Act of 1976 (NFMA). That conversation sparked an idea that quickly caught fire. This chapter documents how LTSP came to be and why four Experimental Forests were central to its success. It began with a ripple effect of the NFMA.

Historical growth plots in the Pacific Southwest

In the past, researchers from the Pacific Southwest Research Station (PSW) undertook forest growth studies to evaluate how best to manage timber resources. However, historical and future data collected at PSW growth plots also have the potential to increase our understanding of the ecological processes occurring in our forests and shed light on national issues of importance. This report provides information on the history, geography, plant species studied, installation, and measurement interval of each plot along with a list of publications arising from data gathered at these plots. This will enable current and future researchers to reidentify these plots and continue research at these locations.